incrementally clean up old RPZ records during updates

incrementally clean up old RPZ records during updates

After an RPZ zone is updated via zone transfer, the RPZ summary
database is updated, inserting the newly added names in the policy
zone and deleting the newly removed ones. The first part of this
was quantized so it would not run too long and starve other tasks
during large updates, but the second part was not quantized, so
that an update in which a large number of records were deleted
could cause named to become briefly unresponsive.

(cherry picked from commit 32da119ed84ad93aa5d754ccc88a3257e2cba10a)